Workers completed the four-storey, 8500 square metre Prince Street complex in just 593 days, or 20 months, since the first sod was turned in January 2019.

The building will house 790 staff from eight government departments.

However, with most staff working from home due to coronavirus restrictions only about 20-25 per cent of them are expected to start work in the building when it opens for business on October 15.

Caroline Myers, the executive director of Property, Fleet and Procurement for the Department of Planning, Industry and Environment, said staff were now packing up at the current DPI building in Kite Street. She said the lease would end on that building on November 3.

Minister for Property Melinda Pavey officially opened the building on Wednesday along with Jenny Armstrong, the wife of former Agriculture minister Ian Armstrong who moved the department to Orange.

Mrs Armstrong said it was a proud day for the family.

"Ian is in care [in Cowra] and I will tell him all about this when I get home," she said.

Ms Pavey said the building was a tribute to Mr Armstrong.

"Ian Armstrong led the charge to relocate almost 500 departmental employees in 1988, the biggest and most successful single departmental relocation in Australian history, bringing public servants closer to the people they serve," she said.

Ms Pavey said the building was modern, sustainable and COVID-safe.

"It's 100 per cent COVID-safe even before we heard that word," she said.

She said the open-plan design and interactive areas would encourage collaboration between departments.

"There is probably not even a government building in Sydney as fabulous as this," she said.

Ms Pavey said the hot-desking design would enable staff on the road to have a base in Orange.

"You don't need a desk at every moment for every person. People come and go, that's what a modern, sustainable workplace is all about," she said.

While Bathurst-based Nationals MLC Sam Farraway and the Nationals member for Cootamundra Steph Cooke were among the official party a notable absentee was the member for Orange Phil Donato, who is not part of the government.

Mr Farraway said he invited Mr Donato via a text message the night before the ceremony but Mr Donato replied the next morning he could not move prior commitments.

The main government tenants at the site will be the Department of Planning, Industry and Environment and the Department of Regional NSW which includes Local Land Services, Department of Primary Industries and the Rural Assistance Authority.

There will also be staff from the Department of Premier and Cabinet and the Department of Education.

Features of the building including solar panels, water harvesting, outdoor areas, two boardrooms with a view to Mount Canobolas and an area for children to be dropped off after school to wait for their parents to finish work and take them home.